The present invention is in the field of multi-outlet structures of the type found in the flow path between coal pulverizing mills and combustion chambers in coal-fired power plants, and in particular found in the classifier cone structure at the upper end of such mills and in the branches of piping between the classifier and the combustion chamber.
In the field of coal pulverizing mills there are generally two types of mills, characterized by the manner in which the pulverized coal is delivered from the mills to a combustion chamber: xe2x80x9csuctionxe2x80x9d mills using exhauster fans to pull the pulverized coal fines from the mill through discharge pipes; and, xe2x80x9cpressurizedxe2x80x9d mills which are fanless and typically entrain the pulverized coal fines in a stream of pressurized air originating at the mill itself.
Each type of mill presents its own problems with respect to the goal of supplying an even, balanced flow of coal fines through multiple pipes to multiple burners in the combustion chamber. In suction mills, for example, the exhauster fan tends to throw coal in an unbalanced stream, with heavier particles settling out to one side of the flow through the pipe and lighter fines on the other. In pressurized mills without exhauster fans, distribution problems tend to occur as a result of the varying lengths of discharge pipe leading from the top of the classifier to the various burners around the combustion chamber. Shorter lengths of discharge pipe generally run rich with air only (but tend to run lean in coal), while longer lengths of pipe tend to run lean in air only (but tend to run rich in coal).
Rich/lean imbalances among the various burners in the combustion chamber produce the usual problems: loss on ignition (LOI) contamination of the ash byproduct; NOX formation; fireball distortion and waterwall erosion; and others known to those skilled in the art.
One common technique for trying to balance coal flow in pipes of different length is known as xe2x80x9cclean air flow testingxe2x80x9d, in which orifice plate restricters are placed in the shorter pipes to try to balance air flow with respect to the longer (slower, lower volume) pipes in an air-only test procedure. The problem with clean air flow testing is that, having balanced air flow in a theoretical test, the introduction of coal fines produces fundamentally different results than the air-only testing would indicate, and the orifice plates worsen distribution problems among and within the pipes. As a result, further efforts have attempted on-line adjustable orificing with coal flow present, with similarly disappointing results.
Another approach to balancing coal flow among multiple pipes has been to use a xe2x80x9cdynamicxe2x80x9d classifier. Dynamic classifiers power-rotate an array of vanes in the classifier cone to decelerate larger particles of coal and encourage lighter fines to travel up and out the classifier into the discharge pipes. It has been found, however, that the use of dynamic classifiers still results in + or xe2x88x9220% differences in distribution among the pipes (resulting in a 40% variance).
The present invention is believed to be the first based on a recognition that redistributing the coal fines immediately adjacent the discharge pipe outlets in a multi-outlet branch structure of the type found at the top of the classifier solves a majority of the downstream distribution problems. In accordance with this recognition, the invention resides in a novel, passive diffuser structure to achieve uniform distribution of coal fines among the individual pipe outlets at the top of the classifier.
In its broadest structural form, the invention is a series of diffuser elements located in the upper end of the classifier, within the cylindrical or annular xe2x80x9cskirtxe2x80x9d usually found surrounding the pipe outlets. The diffuser elements are preferably arranged in concentric rings within the skirt, with a first inner xe2x80x9cringxe2x80x9d at or near an inner surface of the skirt, and a second outer xe2x80x9cringxe2x80x9d arranged at or near an outer surface of the skirt. In a further preferred form, the diffuser elements are circumferentially located both between and aligned with the pipe outlets.
The diffuser elements in a first form comprise rows of serrations or teeth arranged vertically (i.e., generally aligned with the axial flow) with their serrations or teeth projecting into the interior volume of the skirt generally perpendicularly to centrifugal/radial components of the flow. In a preferred, illustrated form they comprise serrated or toothed bars. It will be understood that the terms xe2x80x9cserrationsxe2x80x9d and xe2x80x9ctoothedxe2x80x9d are not intended to limit the invention to any particular geometric form or pattern of the teeth, as they may be pointed, rounded, truncated, squared, etc. They are, however, preferably arranged in alternating high/low patterns along the length of each diffuser element.
In a second form, the diffuser elements comprise horizontally arranged (i.e., generally perpendicular to the axial flow) diffuser elements located in the interior volume of the skirt. The teeth of a horizontal diffuser element are generally perpendicular to axial components of the flow. In the preferred form at least one horizontal diffuser element is a ring diffuser and various combinations of ring diffusers can be placed in concentric relationship to one another. Concentric arrangements of ring diffusers may be in the same plane or may be stacked one above the other in the skirt. The ring diffusers may also be used in combination with the vertical diffuser elements and other horizontal diffuser elements. The ring diffuser element need not be literally a ring, as its geometry will preferably follow that of the pipe or outlet structure, for example in a square configuration.
The inventive diffuser structure is effective not only in multi-outlet classifier skirts, but also in the multi-outlet branch structures found in the piping between the pulverizers/classifiers and the combustion chamber burners. Such multi-outlet branch structures are similar to the multi-outlet classifier skirts, having a pre-outlet volume or plenum which presents a flow path axially head-on to a symmetrical array of individual pipe outlets each having an area significantly smaller than the area of the chamber or plenum feeding the multi-outlet array.
While the present invention is especially designed to provide even distribution to a multi-outlet structure like that in a classifier, it also has utility with riffle-box type pipe junctions and can improve the performance of riffle boxes in providing evenly-split flows to two angled pipes. Likewise, the present invention can be used virtually anywhere in the piping of virtually any coal-fed combustion chamber system (e.g., just prior to the burner itself) where diffusion is critical.
These and other advantages and features of the invention will become apparent upon further reading of the specification in light of the accompanying drawings.